The present invention relates to a method for producing silica glass, particularly silica glass for optical instruments such as optical fiber, by heat-treating porous silica glass, particularly silica gel obtained through the hydrolytic reaction of a starting liquid material.
Hereunder, the background of the present invention will be described by using optical fibers as an example.
Optical fibers are used as a media for various kinds of optical transmissions ranging from optical transmissions of very short distances such as for example in a gastro-camera fiberscope, etc. to optical transmissions of long distances such as are used in optical communication systems. Optical fibers are mainly made of high-silica glass of high purity (if necessary, a dopant is utilized in the glass for adjusting the refractive index thereof).
As methods for producing such optical fibers, there have heretofore been known (i) a method in which a starting material which is molten in a crucible is turned into fiber, (ii) a method in which a high-silica glass film is formed on the inner wall of a silica tube by the CVD process (Chemical Vapor Deposition Process), the tube is collapsed, and the resultant rod is drawn out at a high temperature into a fiber, (iii) a method in which glass soot is prepared by the CVD process, and is piled up and thereafter sintered, and the sintered compact mass is drawn out at a high temperature into a fiber, and the like.
However, these prior art methods are disadvantageous for the following reasons. In case (i), it is difficult to obtain glass having a high silica composition, and the purification of the starting material is difficult because of the inevitable contamination from the crucible, so that optical fibers of high purity are hard to obtain. In case (ii), the mass-producibility is low, a large-sized mother rod is difficult to prepare, and multicomponent glass (glass containing a considerable amount of Na) cannot be produced. Moreover, since a thermal oxidation reaction is utilized, the yield is low when P, Ge, B or the like is used as a dopant element. In addition, the manufacturing equipment is expensive. Also in case (iii), multicomponent glass cannot be produced, the mass-producibility is low, and the manufacturing equipment is expensive. Furthermore, in any of the above cases (i), (ii), or (iii), a glass body in any desired shape cannot be formed. In addition, since both methods (ii) and (iii) produce silicon oxide via the high temperature state and method (i) melts glass sufficiently at high temperature, a high cost is ordinarily unavoidable.
On the other hand, a method capable of producing porous glass at a temperature which is not very high is known. This method is such that alkoxysilanes, or alkoxysilanes to which alkoxides have been added are hydrolyzed to obtain silica gels, or high-silica gels containing the additive oxides.
Further, it has been disclosed that the porous glass made from the silica gel obtained by the hydrolysis of the alkoxysilane can have its temperature raised gradually and heated at 1,000.degree. C. or above in an oxidizing atmosphere, to achieve the desorption of water, the removal of residual organic substances and sintering, thereby turning the porous glass into dense glass (refer to, for example, the official gazette of Japanese Patent Application Laid-open Specification No. 51-34219 and M. Yamane et al.; Journal of Materials Science, vol. 14 (1979), pages 607-611). With such a method, the elimination of the difficulties of the prior art discussed in paragraphs (i), (ii) and (iii) above, can be expected.
However, the dense glass produced by heating the porous glass often forms bubbles when heating it at a high temperature of about 1,300.degree. C. or above in order to work it. Accordingly, the usage of the glass has become extremely limited. In the case of manufacturing optical fiber by employing, as a mother rod, the high-silica glass produced by the above method, when the mother rod is heated to a high temperature in order to turn it into fiber, it forms bubbles and thus is often incapable of providing optical fibers.
Such bubble formation at high temperatures is considered to be caused by residual water, residual OH groups or any other gas component present in the sintered silica glass.
As a method which effectively removes the residual water from porous high-silica glass, there has been known a method in which the glass is exposed to a chlorine-containing atmosphere at a high temperature of 600.degree. C.-1,000.degree. C. and is thereafter sintered at 1,200.degree. C.-1,300.degree. C. (refer to the official gazette of Japanese Patent Application Publication No. 42-23036). According to the inventors' experimental study of this method, the bubble formation in the case of working the porous glass at high temperatures decreased remarkably but could not be completely prevented. At high temperatures of or above 1,300.degree. C., the bubble formation was often noted.
The state of the art is represented by the following references:
(i) The official gazette of Japanese Patent Application Publication Specification No. 42-23036, PA1 (ii) The official gazette of Japanese Patent Application Laid-open Specification No. 51-34219, PA1 (iii) The official gazette of Japanese Patent Application Laid-open Specification No. 53-137086 and PA1 (iv) M. Yamane et al.: Journal of Materials Science, vol. 14 (1979), pages 607-611.